sg: Explicitly define default commandset in config

Created by: mrnugget

Using YAML hackery to define the default commandset can lead to subtle bugs. If someone wants to, for example, overwrite the configuration for the default set in sg.config.yaml, they can't overwrite it under "default" but need to use the name of the actual command set.

Overwriting what the default set is is also a bit of a pain.

This would make it easier, because setting a new default set would just require the following in sg.config.overwrite.yaml:

defaultCommandSet: batches

Question: is this a breaking change?